1. Field of the Invention
The present invention relates to a diamond film thermistor capable of sensing temperature at a high response speed and having an excellent stability against heat, radiation and chemicals.
2. Description of the Prior Art
Ceramics of metal oxides, nitrides and carbides, which have large temperature coefficients of electrical resistance, are used as resistor materials for thin film thermistors. A standard thermistor structure is shown in FIG. 7: a thin resistance film b is formed on an insulating substrate a, metal electrodes c are attached to the thin resistance film b, and lead wires d are connected to the metal electrodes c. The electrical resistances of most ceramic resistors have negative temperature coefficients (their resistances decrease as the temperature increases), and thus the temperature is estimated from the electrical resistance of the resistor.
Japanese Patent Laid-open (Kokai) No. 62-115804 disclosed a thermistor, as shown in FIG. 8, which employed a SiC film as a resistor. The thermistor was constructed by first depositing electrodes c.sub.1 on an insulating substrate a.sub.1, such as alumina, forming a SiC film b.sub.1 on the substrate a.sub.1 by sputtering, and then connecting lead wires d.sub.1 to the electrodes c.sub.1. To protect the SiC film b.sub.1 it was coated with a protective film e.sub.1 (glass) having a specific coefficient of thermal expansion and a specific working temperature.
A thermistor made by a diamond film is described in New Diamond, Vol. 5, No. 2 (see FIG. 9). It is fabricated by depositing a p-type diamond film b.sub.2 on a silicon nitride (Si.sub.3 N.sub.4) substrate a.sub.2 by microwave plasma chemical vapor deposition (CVD) using a source gas consisting of CH.sub.4 and H.sub.2 and containing diborane (B.sub.2 H.sub.6) as a doping agent, forming electrodes c.sub.2 of Au, Mo or Ti, and coating the diamond film b.sub.2 with a protective film e.sub.2 of SiO.sub.2.
Thermistors employing ceramic resistors usually requires a sputtering process for film formation. However, it is difficult to control the stoichiometry of the atomic composition by sputtering. It is also difficult to form a metal resistor uniformly and hence to control the electrical characteristics precisely. Accordingly, the distribution of the characteristics of the thermistors is large, resulting in a low yield of the manufactured thermistors. Furthermore, a deterioration of ceramic thermistor characteristics with time is unavoidable, because the constituent materials of the ceramics is subject to chemical reactions such as oxidation when the thermistor is used, for instance, in the air of steam, and reduction when the thermistor is used in a reducing atmosphere. Therefore, such thermistors become unreliable with time. Regarding the response speed, since ceramics have low thermal conductivities, the ceramic thermistors can not follow a rapid change in the environmental temperature: the response times of ceramic thermistors are in the range of a few seconds or longer.
The SiC thin film thermistor shown in FIG. 8 is less susceptible to the chemical deterioration stated above than ceramic thermistors. However, the thermal conductivity of single crystalline, SiC is as low as 5 W/cm.K, which is lower than that of diamond (20 W/cm.K), and hence the response time of the SiC thin film thermistor is of the order of several tens seconds. On the contrary, the thermal conductivity of diamond thin film is about 10 W/cm.K. Therefore, the response time of diamond thin film thermistors is expected to be about twice better than that of the SiC thin film thermistor. However, the response time of the diamond thin film thermistor, shown in FIG. 9, is in the range of 1 to 2 sec, which is almost comparable to that of the SiC thin film thermistor. This is because (1) the size of the diamond thin film thermistor is as large as 1.5 mm.times.3.8 mm, (2) the substrate is silicon nitride which has a low thermal conductivity, and finally, (3) the temperature sensing area is a diamond thin film without structure.
Furthermore, the diamond thin film thermistor, shown in FIG. 9, has the following disadvantages: Firstly, since the diamond film is deposited directly on the silicon nitride substrate, the density of grain boundaries between individual diamond grains in the film is high in the area along the substrate. Therefore the area has a poor crystallinity, because amorphous carbon and graphite are contained in the grain boundaries. Such grain boundaries permits a rapid deterioration of the diamond film at high temperatures and deprive of its functions.
Secondly, the diamond film and the electrodes in the diamond thin film thermistor of FIG. 9 are formed on an individual substrate. Therefore, the manufacturing process has a low productivity and a high cost.